In the past, there has been used an optical coherent tomography (OCT) apparatus, an example of which is disclosed in Japanese Unexamined Patent Publication No. 2001-79007, and an optical frequency domain imaging (OFDI) apparatus utilizing wavelength sweep, which is an improvement type of apparatus for diagnosing arterioscleosis, for a diagnosis before operation at the time of treatment inside a blood vessel depending on a high functional catheter such as a balloon catheter, a stent and the like, or for a result confirmation after operation Hereinafter, in the present specification, the optical coherent tomography (OCT) apparatus and the optical frequency domain imaging (OFDI) apparatus utilizing wavelength sweep are generically referred to as “optical imaging diagnostic apparatus”.
In the optical imaging diagnostic apparatus, an optical probe unit inserted with an imaging core which is attached with an optical lens and an optical mirror (transmitting and receiving unit) at a distal end of an optical fiber is inserted into a blood vessel, and a radial scan in a blood vessel is carried out by emitting a measurement light into the blood vessel from the transmitting and receiving unit at the distal end while rotating the imaging core and concurrently, by receiving a reflected light from a biological tissue. Then, a coherent light is produced by making the light-received reflected light and a reference light interfere with each other, and a cross-sectional image of the blood vessel is derived based on the coherent light.
Generally, in order to carry out a diagnosis by using a measurement result in such an optical imaging diagnostic apparatus, it is preferable for a user, based on the measurement result, to be able to accurately comprehend the morphological feature of the biological tissues inside the blood vessel and the scattering intensity and the absorption level of the measurement light in each biological tissue.
On the other hand, with respect to the blood vessel cross-sectional image drawn out by a B mode in the optical imaging diagnostic apparatus in the present situation, scattering intensity is expressed as pixel intensity, so that it is possible for a user to comprehend the morphological feature of the biological tissue based on the intensity distribution of the pixel. Also, it is possible to comprehend the scattering intensity of the measurement light in each biological tissue based on the intensity value of the pixel. In addition, in case of an optical imaging diagnostic apparatus, it is possible to draw out a blood vessel cross-sectional image of high resolution, so that it is also possible to distinguish between, for example, tunica intima, tunica media, and tunica adventitia, and further, to identify a plaque or the like.
However, in the case of a blood vessel cross-sectional image developed or generated by the B mode, with respect to the biological tissue which is a biological tissue in which scattering intensity of a calcific plaque, a lipid or the like is low and for which it is necessary to make identification depending on the difference of the absorption level of the measurement light, a situation can arise in which it is difficult to properly identify. On the other hand, in the past, there has been proposed a method in which identification is made based on the sharpness of the intensity change of the pixel in the boundary area of the biological tissues, but in case of a biological tissues in which characteristics of the tissue are mixed up, it sometimes happens even for a well trained expert to make a mistake in the identification by only depending on the method. Consequently, in the optical imaging diagnostic apparatus, there has been desired an improvement of the identification performance with respect to a biological tissue in which it is necessary to make identification depending on the difference of the absorption level.